The next step for Zimmerman will be running on the team's AlterG anti-gravity treadmill, which allows a player to run with a percentage of his body weight taken off, according to Williams. The team will start Zimmerman off light on the AlterG and slowly build him up as part of a "long progression."
"Of course it's encouraging," Williams said. "It's not high stress by any stretch. But it's part of his progression as he works through it. So he'll take the next step and another one and another one and another one."
Williams said he believes the Nationals will see Zimmerman in the lineup before the end of the regular season, a notion general manager Mike Rizzo confirmed Tuesday.
Rizzo said the right-handed slugger could be back after a six-week recovery.
"I would love to have that happen," Williams said. "He's working hard to get back, as hard as he can work at this point. So I would hope so."
Daniel Popper is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.